Height distribution of a surface can be obtained by projecting a light stripe pattern onto the surface and then reimaging the light pattern that appears on the surface. A powerful technique for extracting this information based on taking multiple images (3 or more) of the light pattern that appears on the surface while shifting the position (phase) of the projected light stripe pattern is referred to as phase shifting interferometry as disclosed in U.S. Pat. Nos. 4,641,972 and 4,212,073.
The multiple images are usually taken using a CCD video camera with the images being digitized and transferred to a computer where phase shift analysis, based on images being used as “buckets,” converts the information to a contour map of the surface.
The techniques used to obtain the multiple images are based on methods that keep the camera and viewed surface stationary with respect to each other and moving the projected pattern.
A technique for capturing just one bucket image using a line scan camera is described in U.S. Pat. No. 4,965,665 but not enough information is available to do a phase calculation based on multiple buckets.
Other U.S. patents which show phase shifting include U.S. Pat. Nos. 5,202,749 to Pfister; 4,794,550 to Greivenkamp, Jr.; 5,069,548 to Boehnlein; and 5,307,152 to Boehnlein et al.
U.S. Pat. Nos. 5,398,113 and 5,355,221 disclose white light interferometry systems which profile surfaces of objects.
In the above-noted application, an optical measuring system is disclosed which includes a light source, gratings, lenses, and camera. A mechanical translation device moves one of the gratings in a plane parallel to a reference surface to effect a phase shift of a projected image of the grating on the contoured surface to be measured. A second mechanical translation device moves one of the lenses to effect a change in the contour interval. A first phase of the points on the contoured surface is taken, via a four-bucket algorithm, at a first contour interval. A second phase of the points is taken at a second contour interval. A control system, including a computer, determines a coarse measurement using the difference between the first and second phases. The control system further determines a fine measurement using either the first or second phase. The displacement or distance, relative to the reference plane, of each point is determined, via the control system, using the fine and coarse measurements.